(1) Field of the Invention
The present invention relates to a gradation compensating apparatus for a video signal for improving a television picture quality.
(2) Description of the Prior Art
Recently, gradation compensating apparatus for video signals have been recognized as being important for realizing high picture quality. Specifically, high picture quality is realized by preventing picture suppression and black level buoying. As a result, by improving the picture gradation, the devices have been found to be useful for large color television receivers and for applications where high picture quality is required.
FIG. 1 shows input-output characteristic curves of a black level compensating circuit of a gradation compensating apparatus for a video signal in accordance with the prior art. The abscissa is an input voltage level and the ordinate is an output voltage level. The abscissa marked 1 shows an input voltage level corresponding to the minimum brightness level and the range marked 2 shows an input voltage level corresponding to the brightness of a skin color portion of the input video signal. The curve 3 shows an input-output characteristic without compensation and the output voltage level is proportional to the input voltage level. The curve 4 shows a characteristic with compensation.
In the characteristics of the black level compensating circuit shown in FIG. 1, when the minimum brightness level is as marked by 1, the picture appears as if its black level is buoyant. In order to improve the picture gradation, the minimum brightness level is lowered to a level which is lower than that of the input and there is no level drift due to black level compensation. The slant of the characteristic curve 3 without compensation is made steep and the input voltage level of the minimum brightness level is compensated to approach the pedestal level. Thus, the gradation of the black portion is improved. The higher the average picture level (APL) is, the stronger the compensation is controlled.
However, because only the brightness signal is compensated, as a result, the balance between the color signal and the brightness signal deteriorates. This creates a problem in that the color red and the color of skin become dull. These are colors which are highly sensitive to the human eye, especially in a high APL picture.